Laser systems including fiber amplifiers are commonly used in many applications, including telecommunications applications and high power military and industrial fiber optic applications. For example, both U.S. Pat. No. 5,946,130, issued Aug. 31, 1999 to Rice and U.S. Pat. No. 5,694,408 issued Dec. 2, 1997 to Bott et al. describe many such applications in which laser systems including fiber amplifiers are employed including the processing of materials, laser weapon and laser ranging systems, and a variety of medical and other applications. In this regard, such fiber amplifiers generally include optical fibers that passively transmit optical power, fibers that experience or are designed to enhance performance of a laser through nonlinear optical processes such as Raman-shifting and Brillouin scattering, as well as optical fibers that are doped with a lasing ion embedded in the fiber material. For more details of such applications, see U.S. Pat. No. 5,832,006 issued Nov. 3, 1998 to Rice et al., the contents of which are hereby incorporated by reference in its entirety, which describes coherently phasing Raman fibers in a high brightness array. In addition, see U.S. Pat. No. 6,363,087 issued Mar. 26, 2002 to Rice, the contents of which are also incorporated by reference, which describes a multimode Raman fiber amplifier.
Optical fiber amplifiers are designed to increase the power output levels of the signals propagating therealong. One conventional optical fiber amplifier design is an end-pumped dual-clad fiber, such as that described in U.S. Pat. No. 4,815,079 issued Mar. 21, 1989 to Snitzer et al. Referring to FIGS. 1A and 1B, the dual-clad fiber 1 has a single mode signal core 2, a multi-mode pump core 3 surrounding the signal core, and an outer cladding layer 4 surrounding the pump core for confining pump energy within the pump core such that signals propagating through the signal core are amplified. The signal core will typically be doped with one or more rare earth elements such as, for example, ytterbium, neodymium, praseodymium, erbium, holmium or thulium.
In operation, pump energy is coupled into the pump core 3 at the input end 5 of the fiber. The pump energy then propagates along through the pump core until it is absorbed by the dopant in the signal core 2, thus amplifying signals propagating through the signal core. Although dual-clad fibers 1 can have different sizes, one typical dual-clad fiber includes a signal core that has a diameter of 8–10 μm and a pump core that has cross-sectional dimensions of 100–300 μm. End-pumped dual-clad fiber amplifiers of this size can typically reach fiber energy power levels of 100 W.
In general, laser systems can scale arrays of fiber amplifiers to produce higher power by coupling the output energy from a bundle of relatively low-power, fiber amplifiers. It will be appreciated, then, that scaling fiber amplifiers is generally driven by the ability to coherently combine the output of multiple fiber amplifiers. In this regard, to combine the outputs, the individual fiber amplifiers must typically comprise low-noise, single-mode amplifiers that polarize the output energy such that the energy from the individual amplifiers can be efficiently combined into an integrated array.
Although laser systems generate coherent output power in a manner that is intrinsically efficient, a physical limit referred to as the quantum defect limit typically limits the energy conversion process of such systems. As is known, quantum defect is the difference in the photon energy at which the process is pumped versus the energy of the radiated “lasing” photons. For the most efficient systems, 70% to 90% of the pump energy from laser diodes is converted to output energy. For less efficient arrangements, however, the efficiencies can lie in the range of 40% to 50% or lower. Generally, the quantum defect limit, as well as spontaneous radiation losses, miscellaneous optical absorption losses and other non-productive processes, lead to thermal energy release that heats the fiber amplifier. In this regard, nonlinear, optical processes such as Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS) offer greatly reduced quantum defect. Other nonlinear processes that may operate in a fiber amplifier include self-phase modulation, four-wave mixing, photorefractive conversion and thermo-optical effects (also known as Stimulated Thermal Scattering or STS). Typically, SRS processes are more than 95% efficient, while SBS processes can be more than 99% efficient.
In continuous or quasi-steady operational modes, the temperature of the fiber amplifier will rise until an equilibrium heat transfer condition is established. For high power laser systems, such a rise in temperature can result in elevated temperatures in the core of the fiber amplifier (typically where doped lasing media is located in fiber amplifiers having a doped core). In turn, the elevated core temperature can degrade the efficiency of the laser system, lead to unacceptable optical distortions or, in the extreme, to failure of the fiber amplifiers or surrounding system materials and components. Thus, it would be desirable to design a fiber amplifier and optical fiber laser system that conduct heat away from the core of the fiber amplifier or otherwise decreased the amount of heat generated in the fiber core.
In addition to elevated core temperatures, many conventional fiber amplifiers performing nonlinear processes suffer from localized energy conversion and inefficient conversion of laser power. In this regard, in co-propagated nonlinear processes such as SRS where the pump energy is depleted as a Stokes seed signal is amplified, the fiber amplifier has a conversion profile known as a “lazy-S” profile. In this regard, the conversion rate is related to the product of the pump energy and Stokes signal intensities. Therefore, the conversion rate (versus fiber length) is low when the signal or pump intensity is low (e.g., at the start or near the end of the conversion process). In contrast, the conversion rate is typically the highest when the pump energy and Stokes signal intensities are nearly equal.
The conversion rate typically begins slowly, and then rapidly increases until finally it tapers off once the majority of the pump energy has been depleted. More specifically, after beginning slowly, the conversion rate typically rapidly increases at the fiber length position where the Stokes seed has grown to 20–30% of the pump energy. And once the Stokes signal grows to 70–80% of the original pump energy intensity, the conversion rate tapers off. Such a behavior generally concentrates the majority of the energy conversion and thermal load in a localized section of the fiber amplifier.
For counter-propagated nonlinear processes such as SBS, the pump energy depletion and Stokes signal amplification are inherently competitive. Such behavior can lead to temporal fluctuations (referred to as relaxation oscillations) where the Stokes signal output amplitude can vary dramatically. Depletion of the pump energy due to a strong Stokes signal near the input end of the fiber subsequently decreases the pump energy intensity reaching the output end of the fiber. The decrease in pump energy, in turn, decreases the Stokes signal now propagating in an opposite direction in the fiber, toward the input end. As such, frequencies based upon the speed of light in the fiber and the overall fiber length can develop in the Stokes output signal.
Through proper design, such unstable operating regions can possibly be avoided, but designing around such unstable operating regions presents an additional constraint on the system optimization and operational flexibility of the system. Furthermore, the transient conditions on start-up of the system could pass through such regions of instability, presenting unique design issues and control problems. Thus, it would be desirable to design a fiber amplifier and optical fiber laser system that provides distributed amplification of the lasing power and the output of the nonlinear process that more efficiently converts laser power into a high purity, high coherence optical mode. Likewise, such a system and fiber amplifier would reduce the potential for relaxation oscillations and damaging, transient behavior.